This invention is directed to a method for recovering metal chlorides from a gaseous mixture. More particularly this invention is directed to an improved method for recovering the metal chlorides selectively and without plugging the apparatus in which the metal chlorides are formed, transferred, or condensed.
A common method practiced in the art for separating the metal components of a metaliferous material involves chlorination of the metaliferous material at a temperature sufficiently high to vaporize the major metal chlorides so produced. The gaseous mixture which results is then cooled by a variety of methods to either sequentially or simultaneously condense one or more of the metal chlorides to the solid state while leaving the remaining metal chloride or chlorides in the gaseous state. In this manner it is possible to isolate the desired metal chloride and, if necessary, separately subject it to further processing dictated by the end use requirements.
How efficiently the cooling method effects separation of a particular metal chloride from a gaseous mixture of metal chlorides depends primarily on the particular condensation characteristics of the metal chloride to be separated. Cooling efficiency is particularly critical when the gaseous mixture of metal chlorides contains at least one metal chloride having a wide liquid temperature range, i.e., a wide range of temperature at which the metal chloride is in the liquid state, and a sufficiently high freezing point to allow the formation of solid on the colder surfaces of the apparatus. When gaseous metal chlorides having these characteristics are cooled in the conventional manner, e.g., in spray condensers or by transport through the apparatus, they remain in the liquid state sufficiently long prior to solidification that the liquid droplets coalesce on the apparatus surface and form solid masses. Furthermore, if solid particles are present in the gaseous mixture the coalesing droplets can entrap the particles in the freezing process. The solid masses cannot be pneumatically transported by the gaseous mixture and will eventually plug the apparatus.
For example, when ferrotitaniferous materials are chlorinated, the major chlorides in the resulting gaseous mixture are titanium tetrachloride, ferric chloride, and ferrous chlorides. In practice, the chlorination is conducted at a temperature of about 900.degree.-1100.degree. C. and in that temperature range certain minor metal chlorides such as calcium and magnesium chlorides remain as liquids in the chlorination bed. The minor metal chlorides usually amount to no more than 2% by weight of the ferrotitaniferous material and do not form part of the gaseous mixture leaving the chlorinator. Of the major metal chlorides in the gaseous mixture, it is ferrous chloride which has a wide liquid temperature range, i.e., from 650.degree. to 850.degree. C. and relatively high freezing point, i.e., about 600.degree. C. Consequently, as the gaseous mixture exits the high temperature chlorinator and experiences heat loss, ferrous chloride can begin to liquify at as high a temperature as 850.degree. C. and will remain in the liquid state long enough so that the liquid droplets coalesce and fall on the apparatus surfaces which are below 600.degree. C. where the coalesced liquid freezes. In contrast the ferric chloride component has a very narrow liquid temperature range and will condense to a particulate solid, or "snow out," from the gaseous mixture of a temperature from 200.degree. to 300.degree. C. The particulate solid ferric chloride can be pneumatically transported through the processing apparatus by the gaseous mixture and separated therefrom if desired. The titanium tetrachloride component of the gaseous mixture begins to condense to a liquid at a temperature of less than about 160.degree. C. depending on pressure, but does not freeze until a temperature of less than about -20.degree. C. is reached. Consequentily, even if temperatures of less than 160.degree. C. are reached in the apparatus or on the surfaces of the apparatus, most of the titanium tetrachloride liquid can be pneumatically transported throughout the apparatus and the formation of solid titanium tetrachloride does not occur.
One of the earliest attempts to eliminate ferrous chloride pluggage, described in Groves U.S. Pat. No. 2,999,733 involves chlorinating a material in a reactor at a temperature above 800.degree. C. wherein the temperature of a portion of the vapor space above the chlorination bed is maintained below the temperature at which ferrous chloride is solid. The temperature of the vapor space can be maintained for example by spraying liquid coolant into the vapor space above the chlorination bed in the reaction chamber. A major drawback of this method in practic is the difficulty in maintaining the chlorination bed at the high temperature necessary for the chlorination reaction to take place, i.e., at least 800.degree. C. In addition, a tremendous quantity of heat must be supplied to the chlorinator to maintain reaction temperature resulting in prohibitive energy costs.
A later attempt to eliminate ferrous chloride pluggage is described in Cairns et al. U.S. Pat. No. 3,261,664. This method involves cooling a gaseous mixture of titanium tetrachloride, ferric chloride, and ferrous chloride to a temperature from 500.degree. to 550.degree. C. by, for example, injection liquid coolant, particularly titanium tetrachloride, into the flue cooler into which the gaseous mixture is passed after reaction where liquid can accumulate in the flues.
A relatively recent method for separating titanium tetrachloride from ferric and ferrous chloride, described in Uhland U.S. Pat. No. 3,628,913, involves transferring the gaseous mixture through a duct at a temperature of at least 20.degree. C. above the dew point of ferrous chloride to a spray condenser where the gaseous mixture is cooled to between 150.degree. to 280.degree. C. with liquid titanium tetrachloride. While this method avoids severe ferrous chloride pluggage in the transfer duct, the cooling requires large condensation chamber equipped with a spray disk of sufficient size to atomize large quantities of liquid titanium tetrachloride.
This invention is based primarily on the discovery that gaseous metal chloride, particularly one with a wide liquid temperature range and high freezing point, must be rapidly and thoroughly cooled to a temperature below the freezing point to avoid the formation of liquid phase droplets which can coalesce and form massive solid deposits.